0-length arrays can even be mutably promoted

src/librustc_mir/borrow_check/mod.rs

@@ -1864,8 +1864,12 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                     }),
                 }
             }
-            // promoteds may never be mutated
-            Place::Promoted(_) => bug!("encountered mutable promoted"),
+            // The rules for promotion are made by `qualify_consts`, there wouldn't even be a
+            // `Place::Promoted` if the promotion weren't 100% legal. So we just forward this
+            Place::Promoted(_) => Ok(RootPlace {
+                place,
+                is_local_mutation_allowed,
+            }),
             Place::Static(ref static_) => {
                 if self.tcx.is_static(static_.def_id) != Some(hir::Mutability::MutMutable) {
                     Err(place)

src/test/ui/const-eval/promote_mutable_zst_mir_borrowck.rs

@@ -0,0 +1,17 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+
+#![feature(nll)]
+
+pub fn main() {
+    let y: &'static mut [u8; 0] = &mut [];
+}